In telecommunications facilities, Adaptive Differential Pulse Code Modulation (ADPCM) is used to effectively double the transmission capacity of a Pulse Code Modulation (PCM) system employing, for example, 24 channels of 64 kilobits each channel. An ADPCM system does this by quantizing (compressing) an eight-bit signal sample into, for example, a four-bit sample. A by-product of The ADPCM quantizing process is so-called quantizing noise. The level of quantizing noise that is generated is typically minimal when an ADPCM is processing speech signals. However, in other instances the level of quantizing noise may be significant, such as when an ADPCM is processing, for example, voiceband data signals transmitted by modem equipment. A significant level of quantizing noise occurs because voiceband data transmissions are random, wideband signals, which undermine the capability of the ADPCM quantizing process to effectively predict and adapt to such signals.
Accordingly, there is a need to identify which transmission circuits of a telecommunications network contain ADPCM equipment so that such circuits are not used for the transmission of wideband data signals. To this end, various types of transmission tests have been developed to measure the Signal-to-Noise Ratio (SNR) of a transmission circuit as a way of determining if ADPCM equipment is present in a transmission circuit. One such prior test (disclosed in U.S. Pat. No. 4,768,203) employs a multitone test signal, e.g., a signal composed of twenty-one discrete tones, to detect the presence of ADPCM in a telecommunications network circuit (channel). In this arrangement, the twenty-one discrete tones are combined with a predetermined phase relationship and then transmitted over a target transmission channel. A receiver at the opposite end of the channel filters out the noise signal introduced by the transmission channel by performing a Fast Fourier Transform on the signals that it receives to remove the energy power contained in the appropriate frequency bins. The resulting SNR value of the received signal should then indicate whether the channel contains one or more ADPCMs.
Disadvantageously, the SNR value obtained using twenty-one tones is susceptible to corruption by so-called intermodulation products falling within the frequency bins that contribute to the signal power level. Accordingly, the twenty-one tone measurement has to be performed a number of times to obtain an average SNR value, which, hopefully, represents an accurate SNR value for the channel that is being measured.